The present invention relates to a metal halide lamp, and particularly to a small metal halide lamp which is lighted only by a luminous tube without any outer bulb and which has excellent spectral distribution properties.
Small metal halide lamps, each of which is provided with a luminous tube without an outer bulb, are generally used as light sources for overhead projectors (OHP) and moving picture projectors and are gradually popularized. In each of such small metal halide lamps, the low vapor pressure of the metal halides sealed in the luminous tube is increased by increasing the wall load on the luminous tube so that desired emission can be obtained.
In each of the metal halide lamps having no outer bulb, the wall load on the luminous tube is increased by reducing the size of the luminous tube so that the low vapor pressure of the metal halide is increased. The quartz tube used as the luminous tube is thus devitrified owing to heat or deformed to expand. The metal halide lamps without any outer bulb have a problem with respect to their life which is shorter than that of lamps with outer bulbs.
Of this kind of lamps, dysprosium-thallium (Dy-Tl) lamps have excellent color characteristics such as color temperature and color rendering but show chromaticity coordinates on a X-Y chromaticity diagram (referred to as "chromaticity coordinates" hereinafter) which significantly deviate from the blackbody locus. The Dy-Tl lamps are therefore unsuitable as light sources for overhead projection-type televisions (referred to as "OHP-type TV light source" hereinafter).